An apparatus of this description has been disclosed in our commonly owned U.S. Pat. No. 4,265,771. According to that disclosure, the outlet port is disposed at the periphery of the housing just above the partition in order to drain as much liquid as possible from the filtrate compartment in the purging phase. As long as the liquid in the filtrate compartment fully covers the outlet port, the scavenger gas will accumulate at the top of that compartment and thus exert pressure upon the liquid to drive it out. As soon as the outlet port is unblocked, however, that pressure will greatly diminish whereby some of that liquid will still form a shallow pool above the partition and will flow back into the filter tubes (termed "candles" in our prior patent) when the flow of scavenger gas has terminated. In order to prevent an accumulation of the returning liquid in the tubes and a resulting rewetting of the externally adhering filter cake after the latter has been dried by the gas flow, our prior patent teaches the provision of receptacles for that liquid--termed "return housings"--at the lower ends of the tubes.
Another solution for the problem of backflow is proposed in U.S. Pat. No. 4,289,630 according to which the outlet port is formed by a discharge duct whose entrance lies at the bottom of the compartment and descends from the partition through the filtrand chamber to the peripheral housing wall. In an apparent attempt to minimize the amount of liquid that can accumulate in the filtrate compartment, the latter is made very shallow and is confined by a closed casing of small height which is bodily vibrated by the associated impactor. This vibration, of course, is communicated to the discharge duct which therefore must be specially designed to avoid any contamination of the effluent by the unfiltered slurry.